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A bold line with no triangles: a pressure feature without a front.
Also known as: Trough line, Thick plain line on a weather map
An elongated area of low pressure that is not a boundary between air masses. It is drawn as a plain heavy line precisely because it carries none of the front symbols — the absence is the notation.
An elongated region of relatively low pressure, drawn as a bold line without triangles or semicircles.
Contexts: Maps
Distinguished from a front by carrying no air-mass boundary: the temperature does not change appreciably across a trough, though showers and thunderstorms frequently line up along one.
A plain heavy line is a trough. Adding any marks to it would make it a front, which is a different claim.
A single heavy curved line across a chart, carrying no attached symbols of any kind.